With the rapid development of communication technology, during the process of data communication, the requirement for the capacity of communication is increased. The advancement of optical devices promotes the development of the optical communication technology significantly, and thus high-capacity transmission technology is applied maturely. Wavelength division multiplexing (MDM) technology, as a multiplexing mode for increasing communication capacity in fiber communication, is used more and more widely.
The essence of the WDM technology is as follows: at the transmitter side, each service signal is borne by a different wavelength, and the different wavelength is multiplexed to an optical signal (light wave) by an optical multiplexer, and is transferred to the receiver side through a physical link; at the receiver side, the optical signal of the different wavelength is demultiplexed by an optical demultiplexer, and a different wavelength conversion unit extracts the service signal.
The maximum transmission capacity supported by a WDM system is mainly limited by the following three factors: the service rate that a single wave can bear, the range of wave band, and the wavelength multiplexing density.
As for the first factor, the service rate that the single wave can bear has been increased from 2.5 G to 10 G, and then from 10 G to the present 40 G. However, the frequency of an electronic device itself limits the increase of the rate. Further, the increase of the service rate causes more problems in the transmission process of optical signals, for example, limits such as the Optical Signal to Noise Ratio (OSNR), Polarization Mode Dispersion (PMD), Polarization Dependent Loss (PDL), and dispersion restrict the wavelength transmission distance and narrow the application scope. Therefore, it becomes more and more difficult to improve the transmission capacity of the WDM system by increasing the rate.
As for the second factor, the wavelength range is expanded from the initial C band to the C+L band, and then to the present C+L+S band, and more wavelengths can be accommodated due to the expanded wave band range. However, on the one hand, the transmission property of different wave bands is different; for example, S wave band has high loss and has no matching low-cost amplifier, so it is very difficult to apply the WDM system of the S wave band in practice. On the other hand, strong Raman effect occurs when multiple wave bands are transferred in one fiber, which will cause higher loss of the shortwave band, and impact the overall transmission capacity of the system. Furthermore, demultiplexing and multiplexing of different wave bands often brings additional loss, which will further limit the transmission capacity of the system. Additionally, different wave bands require different amplifiers, so the cost of the system is increased.
As for the third factor, spectrum utilization efficiency of most current WDM systems is lower than 0.1 bit/Hz, and increase of the wavelength division density can improve the spectrum utilization efficiency of a system, thus achieving the purpose of increasing the transmission capacity. However, considering the processing capability of the optical multiplexer and optical demultiplexer, the wavelength interval needs to be determined in the design of the current high-density WDM system and cannot be changed after the design is completed. Although the multiplexer/demultiplexer can be inserted into the optical path by using an optical switch or manually presently to realize the upgrade from a low-density multiplexing system to a high-density multiplexing system, the interruption of services will happen. For the WDM system bearing a large amount of services, the upgrade realized at the cost of interruption of services cannot be accepted by users.
For example, when a system with a wavelength interval of 100 GHz is upgraded to a system with a wavelength interval of 50 GHz, it is necessary to insert a parity interleaved multiplexer/demultiplexer on the line. If the parity interleaved multiplexer is not configured in advance, it is necessary to interrupt the service during the upgrade. Further, when inserting a parity interleaved multiplexer/demultiplexer in the main optical path, as the parity interleaved multiplexer/demultiplexer has a certain loss, the power budget of the entire link needs to be readjusted, which costs too much time, and the 100 G interval wavelength conversion unit also needs to be changed into a 50 G interval wavelength conversion unit, and thus the entire system is all changed actually. As a result, the upgrade from the low-density WDM system to the high-density WDM system is often impracticable.